The decomposition of a free-standing monophase B2 Ni62.5Al37.5 thin film with nanoscale dimensions, for thickness as well as lateral grain size, is investigated by in situ and subsequent high resolution transmission electron microscopy. The atomic structure, microstructure and texture are unchanged until a temperature of 450 degrees C is reached above which a two-phase mixture of B2 + L1(2) is formed. The predominant texture of the B2 film is inherited in the L1(2) grains and even improved during the passage of the transformation front. As a result many small angle tilt boundaries exist in between neighbouring [001] L1(2) grains. Around 1000 degrees C some of the L1(2) grains grow in thickness forming microcrystals and using material from nearby grains thus deteriorating the integrity of the film. Occasionally also fcc Ni microcrystals with up to 15 at.% disordered Al are observed.